1. Field of the Invention
The present invention relates to a thin film transistor formed using an oxide semiconductor layer, and a method for manufacturing the thin film transistor. In addition, the present invention relates to a semiconductor device manufactured using the thin film transistor.
Note that a semiconductor device in this specification indicates all the devices that can operate by using semiconductor characteristics, and electro-optical devices, semiconductor circuits, and electronic appliances are all included in the category of the semiconductor devices.
2. Description of the Related Art
A wide variety of metal oxides exist and are used for various applications. Indium oxide is a well-known material and is used as a transparent electrode material needed for a liquid crystal display and the like.
Some metal oxides exhibit semiconductor characteristics. As metal oxides exhibiting semiconductor characteristics, for example, tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like can be given. References disclose a thin film transistor in which such a metal oxide exhibiting semiconductor characteristics is used for a channel formation region (Patent Documents 1 to 4, and Non-Patent Document 1).
As metal oxides, multi-component oxides as well as single-component oxides are known. For example, InGaO3(ZnO)m (m is a natural number) belonging to homologous series has been known as a multi-component oxide semiconductor including In, Ga, and Zn (Non-Patent Documents 2 to 4).
In addition, it has been confirmed that an oxide semiconductor including such an In—Ga—Zn-based oxide can be used as a channel layer of a thin film transistor (Patent Document 5, and Non-Patent Documents 5 and 6).